222 E. ELowe — Geology of the Isthmus of Panama. 



Culebra Beds. 



From near Empire to Pedro Miguel, a distance of about five 

 miles along the line of the canal through the Culebra cut, are 

 the best exposures of sedimentary rocks in the Canal Zone. 

 The Obispo formation has been uncovered by excavation from 

 JBas Obispo to a point about midway between Empire and 

 Culebra, where it disappears beneath the sediments described 

 by Hill and others as the Culebra beds. A considerable thick- 

 ness of these beds has been exposed in the deepest part of the 

 cut, beneath which borings have shown that they extend to at 

 least 40 feet below sea level. A boring at Kilometer 55 passed 

 through 207 feet of Culebra shales and sandstones without 

 reaching the base of the formation. Nearly 175 feet should 

 be added to this section as representing the part already 

 removed from the canal prism at the point where the boring 

 was made. This would give an observed thickness of nearly 

 400 feet. Hill estimated the thickness as at least 500 feet* 

 and the total is probably greater rather than less. 



So far as they have been exposed or explored by borings the 

 rocks are found to be largely soft shales with abundant sandy, 

 conglomeratic, and calcareous layers. At many horizons lentic- 

 ular bodies of limestone occur. Although there are some thick 

 beds of homogeneous pure clay shales, the majority of the 

 rocks, whether sandstones, shales or conglomerates, are richly 

 carbonaceous ; lens-like seams of lignite have been found at 

 many places in the cut and remains of trees and plants are 

 abundant. The sandstones are impregnated with lime and in 

 the coarser varieties of the rock films of the cementing calcite 

 are readily visible. The conglomerates are composed of sub- 

 angular rock fragments less than a quarter of an inch in 

 diameter held in an impure calcareous matrix. The material 

 of which all of the sedimentary Culebra beds are composed 

 was derived from an older igneous land mass of which presum- 

 ably the Obispo breccia was a part ; quartz is notably absent 

 and the colors range from bluish and greenish gray to nearly 

 black in the richly carbonaceous beds. 



In the upper part of the section at a number of places along 

 the canal cut between Empire and Paraiso are extensive occur- 

 rences of andesitic breccias. Petrographically the fragments 

 composing them closely resemble those of the Obispo breccia, 

 but they are smaller and more uniform in size than those of 

 the Obispo breccias of the same region. In the central area 

 near Culebra summit these breccias are seen to rest upon the 

 shales or fine sandstones of the Culebra beds. At the same 

 locality there are numerous intrusive masses of basalt, and 



*Op. cit., p. 193. 



